Plasma sources are used for the generation of gaseous plasma whose unique physical, chemical, optical, thermal, and biological effects are extensively used in broad areas of science and industry. High-frequency plasma sources utilize radio-frequency or microwave electrical energy to sustain a plasma. High-frequency plasma sources typically include a radio-frequency (RF) shield in order to minimize human exposure to high intensity non-ionizing radiation and reduce electromagnetic interference and power losses due to the radiation of electromagnetic energy. Although plasma is typically produced inside an RF shielded enclosure, the beneficial effects of plasma may be realized either inside or outside the radio-frequency shield.
Plasma sources which use radio-frequency or microwave energy to sustain plasma are usually classified as belonging to one of two broad categories, capacitively coupled or inductively coupled. A capacitively coupled plasma source relies on electrical charges stored on capacitor plates to produce an electric field which accelerates the electrons and ions in the plasma. On the other hand, an inductively coupled plasma source relies on a changing magnetic field, produced by the current flowing through a coil, to induce an electric field in the plasma as described by the Faraday's law of induction. Both capacitively and inductively coupled plasma sources find extensive application in the processing of semiconductor wafers. While the capacitively coupled sources are suitable for producing a uniform low-pressure plasma over a relatively large area inductively coupled sources are capable of producing higher density plasma within a smaller volume. In addition, inductive sources are more efficient in coupling large amounts of electrical power into highly electrically conductive plasma, such as in atmospheric plasma torches which generate very high temperature plasma at atmospheric pressure with many applications in science and industry. The present invention relates to inductively coupled plasma sources. High frequency electrical fields for the generation of plasma may make use of a conductive coil (“field applicator”) driven by an AC current oscillating in the Megahertz to GigaHertz range. A gas within the coil receives energy from the coil through inductive coupling exciting the gas into a plasma state.
Such inductive coupling techniques for generating plasma have a number of significant problems. First, normally the conductive coil must have multiple “turns” and each turn exhibits a mutual capacitance with adjacent turns of the loop creating field (and hence plasma) inhomogenieties which may be manifested as nonuniform plasma ion speeds, trajectories and densities.
Nonuniformities in the plasma may adversely affect applications in which a uniform plasma is required (for example, for etching in the integrated circuit industry) and may waste energy on undesired plasma processes. Since the regions of plasma with higher electron density absorb more power than the regions with lower electron density, the ionization is further enhanced in high density regions and reduced in low density regions, which may lead to instability. The less uniform the electric field, the more likely it is that the plasma will exhibit instabilities ranging from a departure from a local thermodynamic equilibrium to a contraction into a filamentary discharge. Furthermore, a disproportional energy absorption by the plasma in the regions of high field intensity, which are usually located close to the antenna, limits the energy available to other regions of the plasma. The mutual capacitance also limits the voltage that may be applied to the conductive coil without dielectric breakdown between the turns of the coil.
Second, the large amount of electrical power and hence large amounts of electrical current required to pass through the conductive coil produce significant resistive heating requiring complicated or bulky cooling structures. The use of highly conductive materials, such as copper, can reduce resistive losses, but the use of copper and similar metals is complicated by the susceptibility of such highly conductive materials to corrosion and melting in the harsh environment of the plasma.
Third, efficient driving of the conductive loop requires that the loop be part of a resonant structure implemented by placing a tuning capacitor into the coil circuit.
Capacitors suitable for this purpose are expensive and bulky, and the tuning capacitor may require automated control in order to match the differing load when firstly igniting the plasma and then after stable plasma has been formed, adding further cost and complexity.